Professor POU/POE – August 2012

Aug. 13, 2012

This month”s topics: Salt solubility, hardness blending, H2S.

Q: I want my installers and service technicians to have a better understanding of the solubility of salt in water when working on new or used softeners. However, I am confused myself about how much salt there is in a gallon of brine. I have contradictory numbers in my head like 2.6 pounds of salt per gallon of brine and three pounds of salt per gallon of brine. Will you explain this to me? And, what is salometer? I need this for residential, commercial and industrial softeners.


A: The three pounds you”re thinking of is really a rounded-off 2.987 pounds of salt, which is how much salt can be dissolved in a gallon of water at 60° F. Obviously, three pounds is close enough for most work.

Let”s envision a scenario using one gallon of water at an assumed temperature of 60° F.

Fill a pail with the water and add close to three pounds of salt to it. All the salt will dissolve. If we add extra salt it will lie at the bottom undissolved. When three pounds are dissolved, the one gallon starting volume of water will end up being 1.128 gallons of brine, a 13 percent increase in volume. Does the new volume of concentrated brine still contain the three pounds of salt per gallon? No, but the newly expanded 1.128 gallons does. If one gallon of the 1.128 gallons of brine are removed and transferred to another vessel it will contain 2.647 pounds of salt. Hence, one gallon of brine contains about 2.6 pounds of salt and one gallon of water dissolves about three pounds of salt at 60° F. The other 0.3 pounds of salt is in the 13 percent of the brine that we did not remove. It”s still lying there with the extra undissolved salt. This extra salt, by the way, has no impact on the scenario or on any of our calculations.

Note that all of these impressive sounding numbers, extended to three decimal places, are only useful at exactly 60° F. If the temperature is higher, one gallon dissolves more salt. If the temperature is lower, one gallon dissolves less salt. So, the calculations we are taught to use have serious limitations, but never-the-less they are still a good guide. If a water softener is in a basement that almost never gets up to 60° F, some trial-and-error adjustments may have to be made. Or, consider a water softener that is installed outdoors in Florida. The temperature might go from 70° F at night to 110° F at high noon.

Will the brine concentration change from day to night? Our salt solubility theory would say yes, but other factors might come into play like the time of day that the brine is made and whether the salt which is tending to come out of solution when it cools is still readily soluble when it warms.

What about salt-hardening and salt “bridging” which might deter solubility? A hydrometer is a device that floats in a solution in order to determine the solution”s specific gravity. A salometer (older version, salinometer) is a hydrometer that has been calibrated (labeled) to read in units called “degrees salometer.” It is specifically meant to determine the concentration of brine. One-hundred degrees salometer is equal to fully concentrated brine that has the following properties: Specific gravity, 1.2; Baumé degrees, 24.6; NaCl pounds per gallon, 2.647; and gallons of water per gallon of brine, 0.886.

Q: I am wondering how to adjust the final hardness of softened water. The reason for this is that I am installing a condensing water heater that requires that hardness be maintained between 5 and 7 grains per gallon (gpg). I am starting with water that is 13 gpg. How does softening affect TDS?


A: Several of the current softeners have an optional feature whereby the softened and the hard water can be blended. Knowing the model number of the control valve is not enough to determine whether the blending is included because it is often an option.

You should ask your supplier if the softener has a hard water blending or mixing option. Then ask if flow rate or pressure affect the accuracy of the blending.

If the TDS test is done by conductivity there will be a small increase after softening because sodium will conduct slightly greater than calcium or magnesium. If the lab does a TDS test by evaporization there should be no increase. If a water analysis is done for all the cations and anions, and they are converted to a common denominator, they can be summed and there will be no increase.

Q: Professor, I have a well system that uses sulfur water and we have tried everything we could but cannot rid ourselves of the odor and taste issues. The dealer of one well known manufacturer put a whole new system in and it does not work.

Is there an actual way to do this that is not crazy expensive?

Stillwater, NY

A: Unfortunately, there is no single treatment type that works in all cases. The “culprit” you are objecting to is hydrogen sulfide (H2S). This a gas formed from natural sulfate in the groundwater. In most cases, the objective is to oxidize the gas so that in some way it becomes filterable.

In no particular order, oxygen, air, chlorine, ozone, potassium permanganate, treated filter materials or something similar are all used successfully in certain regions of the country. Usually an experienced dealer knows what works best in a particular area. Even if the best available technology is used, there still can be failures due to sizing, incorrect treatment flow rate, incorrect filter backwash rate, chemical feed rate, etc.

I wish I could give you a more definitive answer, but I cannot. I would encourage you, however, to talk to others in your area with the same problem to find out what seems to be working.

David M. Bauman, CWS-VI, CI, CCO, is technical editor of Water Technology® and a water treatment consultant in Manitowoc, Wis. He received his B.A. from the University of Illinois in Industrial Design. He can be reached by email at: [email protected].

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